This invention relates to ultrasound beamformers which receive signals produced by the elements of an ultrasonic array transducer and delay and combine those signals to produce steered and focused ultrasound beams and, in particular, to multiline ultrasound beamformers which produce multiple receive beams or lines in response to a single transmitted wave event.
A number of architectures have been proposed for multiline ultrasound beamformers. The oldest and most basic approach is to use multiple single line beamformers which are operated in parallel, as shown in U.S. Pat. Nos. 4,644,795 (Augustine) and 5,318,033 (Savord). In such an arrangement each element of the transducer array is connected to a channel of each of the beamformers. Each of these channels applies delays to the signals from the element which are appropriate to steer and focus the beam being formed by the beamformer of which that channel is a part. The signals delayed by each channel of a beamformer are combined to form a uniquely steered and focused beam, and the multiple beams produced simultaneously by the parallel operated beamformers are used to form multiple lines of an ultrasound image.
A problem presented by this approach is the hardware and operational complexity of multiple parallel beamformers. A conventional single line beamformer for a 128-element transducer array has 128 individual processing channels which are coordinately operated, a complexity unto itself. When two such beamformers are operated in synchronism the hardware and control complexity approximately doubles. Consequently, the cost of such an approach can be significant.
Another architecture which addresses this problem is one which integrates the processing of multiple beams within each channel of the beamformer. An effective approach to integrated processing of multiple beams is to time multiplex the processing of the different beams within a channel as shown in U.S. Pat. No. 5,469,851 (Lipschutz). In a time multiplexed beamformer successive samples of each beam are time interleaved in a pipelined data stream. The processors in the channel rapidly alternate between the characteristics needed to process each beam and thereby process the successive samples each according to the requirements needed to form one of the beams. At the end of the process the samples of the beams are combined with the corresponding beam samples of the other channels to produce multiple time-interleaved beam samples. The beam samples are thereafter sorted out and used to form lines of an ultrasound image.
A drawback of this approach is that the pipelined data path which conducts the samples is necessarily band-limited. The samples in a sampled-data system must satisfy the Nyquist criterion so that the sought-after echo data can be accurately recovered without aliasing. The data must furthermore be handled and processed in real time. These demands mean that the system designer must make compromises when designing such a system. In particular the designer must trade off the bandwidth or frequency of the transducer with which the multiline beamformer may operate in consideration of the sampling rate of the system as the number of beams being processed increases. As a result the beamformer is restricted to operation with only low frequency transducer probes when beamforming a significant number of beams. Furthermore, variable passband filters and decimation are needed to restrict the bandwidth of the received echo signals in each channel when higher order multiline is called for, which further increases the cost and complexity of the beamformer. It would be desirable to have a multiline beamformer architecture which does not require tradeoffs affecting performance during higher order multiline operation, and which obviates the need for variably controlled and decimating filters.
In accordance with the principles of the present invention, a digital multiline beamformer is provided which produces multiple receive beams in response to a single transmit event. An embodiment of the present invention includes a bulk delay which provides a delay applicable to all of the beams received at one time, and generally provides a common steering delay. The bulk delay is succeeded by parallel filter processing paths which produce the multiple beams simultaneously. The filter processing paths are capable of producing output samples of each beam at a fixed output data rate, regardless of the number of beams currently being processed. In various ones of the illustrated embodiments the filter processing paths utilize FIFO or delay registers and share or multiplex different taps of the delay registers. In other embodiments a single filter processing path produces the samples for multiple parallel beam paths.